As the society becomes more information oriented, there is an increasing demand for display devices of higher-definition and larger display area for AV equipments such as TVs, monitors for OA equipments. Examples of such display devices include CRTs (Cathode-Ray Tube) displays, LCDs (Liquid Crystal Display), plasma displays, EL (Electro Luminescent) displays, LED (Light Emitting Diode) displays, etc.
For the described display devices, developments of a larger display area have been made for practical applications. However, as the display area of the display device is upsized, an accompanying increase in weight, size and power consumption of the display device is expected. Therefore, when upsizing display area, efforts to achieve characteristics of lighter weight, thinner and lower power consumption are required at the same time.
Among the described display devices, the use of the liquid crystal displays has achieved wide-spread acceptance in a variety of applications in various fields recently. The LCDs have an advantage over other display devices in that they can be installed even in a small space with ease because of its beneficial characteristic of lighter and thinner structure compared with other display devices and requires small power consumption. Moreover, as the LCDs are evolvable into full-color display with ease, they can be suitably applied to display devices of a large-sized display area such as large-sized monitors, wall hung type display devices. Thus, the LCDs are the best candidate for large-sized display area.
However, when an attempt is made to upsize the display area of the liquid crystal display, or improve the resolution, the LCDs have the following drawbacks:
1 A cost of the LCDs increases as the yield is lowered due to a disconnection of a signal line, pixel defect, etc., experienced in the manufacturing processes; and
2 Even when adopting the active matrix driving system which shows an excellent response characteristic, a significant delay in a gate signal can not be avoided, which definitely impairs display performances.
As a solution to the described problem 1, a method of realizing a large-sized display area by adopting one large LCD panel prepared by connecting a plurality of LCD panels have been proposed. For example, inventors of the present application proposes a liquid crystal display device (Japanese Unexamined Patent Publication No. 122769/1996 (Tokukaihei 8-122769)) which adopts a liquid crystal display panel of a new multi-panel system in which joints of panels are not noticeable. The described liquid crystal display panel realizes LCDs which permit a natural image to be displayed on a large display area at low cost.
On the other hand, as a solution to the described problem 2, a method of reducing the resistance of the gate wirings by increasing the film thickness of the gate wirings has been proposed. For example, in the case of liquid crystal display panels for notebook type PCs, the film thickness of the gate wiring is increased from around 0.3 .mu.m to not less than 0.5 .mu.m. By increasing the film thickness of the gate wirings in the described manner, a delay in signal can be suppressed.
The method of increasing the film thickness of the gate wirings will be explained in detail.
For example, as shown in FIG. 8, each display pixel 21 of a color liquid crystal display panel 20 is composed of three sub-pixels, i.e., a red pixel 22, a green pixel 23 and a blue pixel 24. Each sub-pixel is formed by laminating a pixel electrode formed on an active matrix substrate (to be described later) and a color filter formed on the color filter substrate to be fit in a shape of the pixel electrode in each color.
The active matrix substrate is prepared by forming an active element on a transparent substrate. On the other hand, the color filter substrate is prepared by forming a color filter, etc., on the transparent substrate so as to face the active matrix substrate as in the case of the active matrix substrate.
Between the active matrix substrate and the color filter substrate which constitute the liquid crystal panel 20, a liquid crystal is sealed. The color of each display pixel 21 is determined by adjusting a light transmitting through the liquid crystal. Each sub-pixel is enclosed by a black matrix 25 to prevent light from entering therein from other regions. On the other hand, to allow a transmissive light to be transmitted therethrough without being disturbed, gate wirings and source wirings are formed with respect to TFT (Thin Film Transistor) element which is the active element for driving the pixel electrode.
The display pixel 21 of the liquid crystal display panel 20 as shown in FIG. 8 is constituted by a wiring pattern of the active matrix substrate (hereinafter referred to as a TFT substrate) 26 as shown in FIG. 9. The TFT substrate 26 is placed so as to face the color filter substrate, and the TFT element is formed as an active element.
Each pixel electrode 27 provided for each color formed on the TFT substrate 26 is enclosed by a source wiring 28 for supplying a data signal to the pixel electrode 27, and a gate wiring 29. It is controlled such that the pixel electrode 27 is driven by a TFT element 30 provided in a vicinity of a crossover A between the source wirings 28 and the gate wirings 29.
The source wiring 28 and the gate wiring 29 cross at the crossover A via a gate insulating film 31 (to be described later). For example, in the case of a liquid crystal display panel 20a wherein the TFT element 30 of the display pixel 21 has an inverse stagger structure, a gate insulating film 31 is formed on the gate wiring 29, whereon the source wiring 28 crosses via the gate insulating film 31.
As described, when an attempt is made to increase the film thickness of the gate wiring 29 to prevent a delay in the gate signal, a level difference of the source wiring 28 at the crossover A becomes greater, thereby presenting the problem that a wiring disconnection of the source wiring 28 occurs at the crossover A at a higher rate.
Here, the film thickness of the gate wiring 29 is increased according to the size of the entire liquid crystal display panel as a final product. Therefore, the described problem of a higher rate of the wiring disconnection arises not only when realizing liquid crystal display panels of an upsized display area by a piece of large panel, but also when realizing them using the multi-panel system. Therefore, a higher rate of wiring disconnection cannot be avoided when upsizing the LCD panel.
Additionally, for not only the upsized LCD panels but also for many LCD panels, the redundant structure which permit the disconnection inferior of the source wiring 28 to be fixed by a projection of a laser beam has been generally adopted. However, the such redundant structure has the drawbacks in that the number of the source wirings that can be fixed is limited, or a cost increases due to an increase in the number of parts to be fixed by a projection of a laser beam. Therefore, the described LCD panels of a large display area which has quite a few number of display pixels does not offer a fundamental solution to prevent a disconnection of the source wiring 28.